System and Method For Protecting Transmissions of Wireless Microphones Operating in Television Band White Space

ABSTRACT

A wireless microphone system broadcasts a pilot tone at the designated ATSC pilot position in the TV Band channel being used by the wireless microphone system. The pilot tone is a readily detectable waveform transmitted in the ATSC pilot position. The pilot tone can be generated by any one of: a standalone pilot tone generator; a pilot tone generator incorporated into a wireless microphone receiver; or, a pilot tone generator incorporated into a wireless microphone.

RELATED APPLICATIONS

This is a continuation application of U.S. application Ser. No.12/779,782 filed on May 13, 2010, the entirety of which is incorporatedherein.

FIELD OF THE INVENTION

This invention relates in general to the secondary use of television(TV) band channels and, in particular, to a system and method forprotecting the transmissions of wireless microphone systems operating inthe television band (TV Band).

BACKGROUND OF THE INVENTION

The opening of available TV Band spectrum for usage by secondary TV Banddevices has created a need for efficient spectrum sensing mechanismsthat can reliably detect available TV Band white spaces to ensure thatprimary users such as digital television (DTV) broadcasters and wirelessmicrophones are protected from interfering broadcasts by the TV Banddevices.

Sensing available white spaces in the VHF/UHF bands is vital to theoperation of secondary TV Band devices. Protection of primary incumbentoperators like digital television (DTV) stations and wireless microphoneoperators is mandated by the United States Federal CommunicationsCommission (FCC) and other federal authorities around the world. The DTVand wireless microphone sensing requirements set forth by the FCC arevery stringent, requiring a sensing threshold setting of −114 dBm.Acquiring and sensing signals from VHF/UHF channels at the requiredsensing threshold are very challenging. None of the known technologiescurrently being tested are capable of meeting that sensing threshold.

IEEE 802.22.1 proposed that every wireless microphone transmit a“beacon” in the form of a direct sequence spread spectrum signal toindicate that the wireless microphone is in operation. This solutionrequires that each TV Band device be equipped with a complexreceiver/detector capable of detecting the spread spectrum beaconsignal. This solution will therefore increase the cost of both TV Banddevices and wireless microphone transmitters.

As understood by those skilled in the art, an Advanced TelevisionSystems Committee (ATSC) TV signal is created using an MPEG encoder toconvert digitized video data into a high speed (19.39 Mbits/second) bitstream, which is passed to a DTV circuit that adds framing informationand randomizes the data to “smooth” it out. The framed data stream isthen encoded and a series of synchronization signals is inserted into it(Segment-sync, Field-sync, and ATSC Pilot) to provide a signal that isapplied to an 8-VSB (8-Level-Vestigial Side Band) modulator, whichoutputs a baseband DTV signal. Finally, the baseband DTV signal is mixedwith a carrier signal to “up-convert” it to a desired channel orfrequency. The up-converted DTV signal typically occupies 5.38 MHzspectrum, thereby being confined to within 90% of the 6 MHz DTV Bandchannel allocation.

The ATSC pilot is a constant DC value at the baseband and becomes asingle tone at radio frequency (RF) occupying a fixed location withinthe transmitted 6 MHz channel. The ATSC pilot has constant amplitude(normalized value of 1.25 on the 8-Level-Vestigial Side Band scale). So,if an ATSC pilot is detected, the channel is declared occupied by a DTVsignal. If the DTV signal is denoted by sTV(t), the transmitted signaltTV(t) includes sTV(t), and the ATSC pilot sPilot(t). The signalreceived by a white space detector, denoted by r(t), includesα{sTV(t)+sPilot(t)}, where α is a factor that represents impairmentsintroduced by the communication channel.

The ATSC pilot can be detected, for example, if the RF signal receivedfrom a selected DTV channel is frequency converted to baseband andsubsequently narrowband filtered around the DC component thereof, in amanner well known in the art. The energy in the filtered signal can beanalyzed to determine the presence, or absence, of the ATSC pilot, whichprovides a reliable indication of the presence, or absence, of a DTVsignal.

Alternatively, the signal received from the selected DTV channel can befiltered and frequency translated to an intermediate frequency band. Thepresence, or absence, of the pilot signal can then be determined byaccumulating and analyzing the received signal energy within the narrowfrequency band that the DTV pilot signal is mandated to occupy.

A wireless microphone (WM) normally transmits a frequency modulated (FM)waveform s_(mic)(t), which can be mathematically expressed as:

s _(mic)(t)=A cos(2πf _(m) t+2πk _(m)∫_(−∞) ^(t) m(x)dx)  eq. 1

Where: A is the amplitude; fm is the carrier frequency; m(x) is themessage signal; and, k_(m) is a scaling factor.

The wireless microphone FM waveform has no known sequence or pattern tofacilitate sensing. Most wireless microphone signals occupy a bandwidthof 200 kHz or less. As understood by those skilled in the art, the FMwaveform may swing back and forth within the 200 kHz bandwidth.Furthermore, wireless microphone receivers need to be protected frominterference from signals transmitted by TV Band device transmitters. Awireless microphone typically requires a desired-to-undesired signalpower ratio of 20 dB or more for operation. Therefore, in order to keepinterference from TV Band device transmitters within acceptable limits,wireless microphone transmissions will have to be detected at levelseven lower than the sensing threshold of −114 dBm. Consequently,reliably detecting and protecting wireless microphone systems isextremely difficult.

There therefore exists a need for a viable way to reliably detect thepresence of a wireless microphone system at reasonable cost.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a system and amethod for protecting the transmissions of wireless microphonesoperating in a TV Band channel.

The invention therefore provides a method of protecting transmissions ofa wireless microphone system operating in a television (TV) bandchannel, the method comprising: broadcasting a pilot tone from thewireless microphone system, the pilot tone being broadcast within afrequency band of the TV Band channel used by the wireless microphonesystem at a location in the frequency band mandated for an AdvancedTelevision Systems Committee (ATSC) pilot for a digital television (DTV)broadcast, to permit a TV Band device to detect the pilot tone in orderto determine that the TV Band channel is in use by the wirelessmicrophone system.

The invention further provides a method of determining whether a TV Bandchannel is available for use by a TV Band device, comprising: tuning tothe TV Band channel; down-converting a radio frequency signal receivedon the TV Band channel to yield a down-converted signal; performing apredetermined number of Fast Fourier Transforms (FFTs) on thedown-converted signal and accumulating an energy of respective binsassociated with the respective FFTs; selecting a predetermined number ofFFT bins centered at a mandated location of an Advanced TelevisionSystems Committee (ATSC) pilot in the TV Band channel; and analyzing theselected FFT bins to determine if an energy spike is present in any oneof the selected FFT bins.

The invention yet further provides a wireless microphone system,comprising: a pilot tone generator that generates a pilot tone andbroadcasts the pilot tone within a frequency band of a TV Band channelused by the wireless microphone system; wherein the pilot tone isbroadcast at a location in a frequency band designated for an AdvancedTelevision Systems Committee (ATSC) pilot in the TV Band channel, topermit a TV Band device to detect the pilot tone in order to determinewhether the TV Band channel is in use by the wireless microphone system.

The invention yet further provides a wireless microphone receiver,comprising a pilot tone generator adapted to generate a pilot tone of apredetermined waveform and to broadcast the pilot tone at a frequencylocation designated for an Advanced Television Systems Committee (ATSC)pilot in a TV Band channel used by a wireless microphone system withwhich the wireless microphone receiver is associated.

The invention yet further provides a pilot tone generator for a wirelessmicrophone system that generates a pilot tone of a predeterminedwaveform and broadcasts the pilot tone at a frequency location mandatedfor an Advanced Television Systems Committee (ATSC) pilot in a TV Bandchannel used by the wireless microphone system.

The invention yet further provides a wireless microphone of a wirelessmicrophone system, comprising: a pilot tone generator adapted togenerate a pilot tone of a predetermined waveform at a frequencydesignated for an Advanced Television Systems Committee (ATSC) pilot ina TV Band channel used by the wireless microphone system; a signalcombiner that combines the pilot tone with a frequency modulation (FM)signal generated by the wireless microphone to create a combined signal;and a combined signal transmitter that broadcasts the combined signalreceived by a wireless microphone receiver of the wireless microphonesystem.

The invention yet further provides a TV Band device, comprising a pilotsensor adapted to detect a predetermined waveform broadcast by awireless microphone system at a frequency location designated for anAdvanced Television Systems Committee (ATSC) pilot in a TV Band channelused by the wireless microphone system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only and withreference to the following drawings, in which:

FIG. 1 is a schematic diagram of a wireless microphone system inaccordance with one embodiment of the invention;

FIG. 2. is a schematic diagram of a wireless microphone system inaccordance with another embodiment of the invention;

FIG. 3 is a schematic diagram of a wireless microphone system inaccordance with yet a further embodiment of the invention;

FIG. 4 is a schematic diagram illustrating an apparatus for generating apilot tone at the ATSC pilot position in accordance with one embodimentof the invention;

FIG. 5 is a schematic diagram illustrating an exemplary circuit forimplementing the apparatus shown in FIG. 4;

FIG. 6 is a schematic diagram illustrating an apparatus for generatingthe pilot tone at the ATSC pilot position in accordance with anotherembodiment of the invention;

FIG. 7 is a schematic diagram illustrating an exemplary circuit forimplementing the apparatus shown in FIG. 6;

FIG. 8 is a schematic diagram illustrating certain components of awireless microphone in accordance with one embodiment of the invention;

FIG. 9 is a flow chart illustrating a method of sensing TV Band channelsin accordance with the invention to find a white space channel that isavailable for use by a TV Band device;

FIG. 10 is a schematic diagram of a pilot sensor of a TV Band device inaccordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a system and a method of protecting wirelessmicrophone systems that use television band channels. In accordance withthe invention a wireless microphone system broadcasts a pilot tone atthe designated ATSC pilot position in the TV Band channel being used tobroadcast the wireless microphone FM signal. The pilot tone may be asimulated ATSC pilot or another easily detectable waveform transmittedin the ATSC pilot position. The pilot tone can be generated by any oneof: a stand-alone pilot tone generator; a pilot tone generatorincorporated into a wireless microphone receiver; or, a pilot tonegenerator incorporated into a wireless microphone. If a stand-alonepilot tone generator is used, the wireless microphone requires nomodification and the wireless microphone receiver only requires anaddition of a mechanism, a band pass filter for example, to eliminatethe pilot tone from the received signal. This significantly simplifiesthe detection of the presence of a wireless microphone system by TV Banddevices, and helps ensure that wireless microphone systems are protectedfrom interfering TV Band device transmissions. Cost is reduced becauseincumbent white space sensor circuits in TV Band devices can be used toreliably detect wireless microphone transmissions without modification.

FIG. 1 is a schematic diagram of a wireless microphone system 10 inaccordance with one embodiment of the invention. In this embodiment, thewireless microphone system (WMS) 10 includes a wireless microphone (WM)12 of any known type, make or origin. The WM 12 has an antenna 14 thattransmits an FM signal 16 in a manner well known in the art. Asunderstood by those skilled in the art, the antenna 14 may be aninternal or an external antenna. The WM 12 typically transmits the FMsignal 16 at about 10 mW transmit power. The FM signal has a coveragearea 18 that generally gives the WM 12 an effective range of about 100m, depending on environmental factors.

A pilot tone generator 20, which generates a pilot tone (a simulatedATSC pilot or a similar easily detected waveform) and broadcasts thepilot tone at the designated ATSC pilot position, i.e. 309.441 kHz fromthe lower frequency edge of the 6 MHZ DTV Band channel assigned to thewireless microphone system 10. In one embodiment of the invention, thepilot tone generator 20 uses a modulating sequence to generate the pilottone at radio frequency (RF), as will be explained below with referenceto FIGS. 4 and 5. The pilot tone generator 20 has an antenna 21 thatbroadcasts the pilot tone 22. The transmit power used to broadcast thepilot tone 22 is regulated by FCC 47 C.F.R. Part 74, which dictates thatthe transmit power cannot exceed 50 mW in the VHF band or 250 mW in theUHF band. Thus the transmit power of the pilot tone generator 20 can beregulated so that the coverage area 24 of the pilot signal is greaterthan the coverage area 18 of the WM 12. Consequently, a TV Band device32 with a pilot sensor 34 within the coverage area 24 can detect thepilot tone 22 and determine that the TV Band channel used by the WM 12is occupied, even if the TV Band device 32 cannot detect the FM signal16.

A wireless microphone receiver (WM RX) 26 of the WMS 10 in accordancewith the invention receives the FM signal 16 and the pilot tone 22 viaan antenna 28. A pilot tone reject mechanism 30, a band pass filter forexample, removes the pilot tone 22 from the received signal, which isthen processed in a normal manner well known in the art. The WM 12 cantransmit the 200 kHz FM signal anywhere within the 6 MHZ TV Band channelexcept the bandwidth occupied by the pilot tone.

FIG. 2 is a schematic diagram of a WMS 40 in accordance with anotherembodiment of the invention. The WMS 40 is similar to the WMS 10described above. However, a WM RX 42 includes a pilot tone generator 44that generates the pilot tone. The pilot tone 48 is broadcast by anantenna 46 at a transmit power that gives the pilot tone a coverage area50, which may be larger than the coverage area 18 of the WM 12. The WMRX 42 also has a receive antenna 54 which receives the FM signal 16 aswell as the pilot tone 48, and a pilot tone reject mechanism 52 thatremoves the pilot tone 48 from the received signal, which is thenprocessed in a normal manner well known in the art and explained above.

FIG. 3 is a schematic diagram of a WMS 60 in accordance with yet afurther embodiment of the invention. In this embodiment, a WM 62includes a pilot tone generator 64 that generates the pilot tone, whichis combined with the FM signal as will be described below in more detailwith reference to FIG. 8. The combined signal 66 is broadcast by anantenna 68, which may be an external or an internal antenna of any typeknow in the art. The combined signal is broadcast at a transmit power ofup to 50 mw, for example, to provide a range of coverage 70. The WM RX26 in the WMS 60 is the same as the one described above with referenceto FIG. 1.

FIG. 4 is a schematic diagram illustrating an apparatus for generating apilot tone at the ATSC pilot position in accordance with one embodimentof the invention. The pilot tone is defined at baseband by the followingsequence:

{b}={M,−M,M,−M,M,−M,M . . . }  eq. 2

Where, M is a constant value whose sign is changed according to apre-determined sequence every Tp second. The value of Tp, and thepattern of the sign change are design parameters that are respectively amatter of design choice. In accordance with one embodiment of theinvention, M=1, which is useful for differentiating a pilot tonebroadcast by a wireless microphone system from the pilot tone of a DTVbroadcast.

In accordance with the invention, when a wireless microphone site is inoperation one of: the pilot tone generator 20 (FIG. 1); the wirelessmicrophone receiver 42 (FIG. 2); and, the wireless microphone 62 (FIG.3) transmits a pilot tone modulated by sequence {b} at the ATSC pilotposition within the TV Band channel being used for the FM transmissionsof the wireless microphone(s) in the wireless microphone system.

If the ATSC symbol rate is denoted as f_(sym):

$\begin{matrix}{f_{sym} = {{4.5 \times \frac{684e\; 06}{286}} = {10.762\frac{Msym}{s}}}} & {{eq}.\mspace{14mu} 3}\end{matrix}$

Denoting the symbol period

$T_{sym} = \frac{1}{f_{sym}}$

the ATSC symbol stream out of the bits-to-symbol mapper is representedas:

a(kT _(sym)), k=0,1,2,3, . . .   eq. 4

In the ATSC transmitter, a pilot DC value is added to this sequence asfollows:

c((kT _(sym)))=a((kT _(sym)))+1.25, k=0,1,2,3, . . .   eq. 5

The sequence c(kT_(sym)), is fed to a Vestigial Side Band (VSB)modulator and the constant DC value of 1.25 becomes the DTV pilot at theRF output.

As explained above, in accordance with one embodiment of the inventionthe pilot tone broadcast by a wireless microphone system is defined by asequence b(kT_(sym)), k=1, 2, 3, . . . with the followingcharacteristics:

$\begin{matrix}{{b\left( {\lbrack{kT}\rbrack_{\downarrow}{sym}} \right)} = {M \times {{sign}\left( {{\sin \left( {2 \times \pi \times f_{\downarrow}p \times \lbrack{kT}\rbrack_{\downarrow}{sym}} \right)},{k = 0},1,2,3,{\ldots {Where}},{{f_{p} = {{\frac{1}{T_{p}}\mspace{14mu} {with}\mspace{14mu} T_{p}}T_{sym}}};}} \right.}}} & {{eq}.\mspace{14mu} 6}\end{matrix}$

and

-   -   T_(p) represents the period of the alternating +M, −M, +M, −M .        . . pattern.

In one embodiment, the period T_(p) of the alternating sequenceb(kT_(sym)), is assigned such that the bandwidth of the pilot tone ismaintained within a band of 30 kHz centered on the designated ATSC pilotposition. It should be noted however that it is only important that themodulated pilot tone is maintained within a narrowband centered on thelocation of an Advanced Television Systems Committee (ATSC) pilot for aDTV broadcast.

As shown in FIG. 4, this modulating sequence for generating the pilottone is passed to a simplified ATSC transmitter embodied in the pilottone generator 20 or the wireless microphone receiver 42. The simplifiedATSC transmitter includes a VSB modulator 80, which generates thevestigial sideband signal using the modulating sequence (+M, −M). Aradio frequency (RF) up-converter 82 receives the output of the VSBmodulator 80 and adds the output to a carrier frequency to up-convert itto the frequency of the TV Band channel assigned to the wirelessmicrophone system. The up-converted signal is broadcast using an antenna84.

In another embodiment of the invention, the pilot tone can be producedin either the analog or the digital domain and up-converted to thedesignated pilot frequency within the TV Band channel. As one example, anumerically-controlled oscillator (NCO) can be programmed to output asine wave at the frequency f_(IF). The output of the NCO is multiplieddigitally by the +M, −M (+1, −1) sequence by a digital multiplier. Thedigital multiplier output is processed by a digital-to-analog converter(DAC) and subsequently up-converted in the analog domain to thedesignated pilot frequency within the TV Band channel assigned to thewireless microphone system.

FIG. 5 is a schematic diagram illustrating an exemplary circuit forimplementing the apparatus shown in FIG. 4. The pilot tone is generatedat the ATSC pilot position using the Weaver single side band generationtechnique. The modulating sequence b(kT_(sym)), k=0, 1, 2, 3, . . . isfed via respective signal lines 100 a, 100 b to respective digitalsignal multipliers 102 a, 102 b. The digital signal multiplier 102 amultiplies the modulating sequence by a first output 106 a of anumerically controlled oscillator (NCO) 104. The first output 106 a ofthe signal generator 104 is represented by:

$\begin{matrix}{{\cos \left( {\omega_{d}{kT}_{sym}} \right)}{{{Where}\text{:}\mspace{14mu} \omega_{d}} = {2 \times \pi \times \frac{{ATSC\_ Symbol}{\_ Rate}}{4}}}} & {{eq}.\mspace{14mu} 7}\end{matrix}$

The signal multiplier 102 b multiplies the modulating sequence by anoutput 106 b of the NCO 104. The output 106 b is represented by:

sin(ω_(d) kT _(sym))  eq. 8

The outputs of the respective signal multipliers 102 a and 102 b arepassed to respective up-sampling circuits 108 a and 108 b, whichrespectively up-convert the signals using an appropriate up-conversionratio selected in a manner known in the art. In one embodiment theup-sampling ratio N₁=2. The respective signals are then passed torespective low pass filters (LPFs) 110 a and 110 b which respectivelyremove unwanted high frequencies from the respective up-sampled signalsbefore they are passed to respective second up-sampling circuits 112 a,112 b which up-convert the signals using a second up-sampling ratio N₂.In one embodiment, N₂=4. The up-converted signals are then passed torespective low pass filters 113 a, 113 b which remove unwanted highfrequencies from the further up-sampled signals before they are passedto respective digital-to-analog (DAC) converters 114 a, 114 b whichconvert the respective digital signals to corresponding analog signalsin a manner well known in the art. The respective analog signals arepassed to respective low pass filters 115 a, 115 b to remove unwantedfrequencies before they are passed to analog signal multipliers 120 a,120 b. The analog signal multiplier 120 a multiplies the analog signalby:

cos(ω_(c) t)  eq. 9

-   -   where: ω_(c)=2×π×f_(c), and f_(c) is the center frequency of the        DTV Band channel in use by the WM transmission system.

The analog signal multiplier 120 b multiplies the analog signal by:

sin(ω_(c) t).  eq. 10

The output of the analog signal multiplier 120 a is passed via 122 a toan analog signal summer 124. The output of the analog signal multiplier120 b is passed via 122 b to the analog signal summer 124. The analogsignal summer 124 outputs the RF signal to the antenna 84 shown in FIG.4.

FIG. 6 is a schematic diagram illustrating an apparatus for generatingthe pilot tone at the ATSC pilot position in accordance with anotherembodiment of the invention. As shown in FIG. 6, a modulating sequenceb(kT_(s)) is input to a digital waveform generator 150, which will beexplained below with reference to FIG. 6. The modulating sequenceb(kT_(s)) is defined by:

b(kT _(s))=M×sign(Sin(2πf _(p) kT _(s))), k=0,1,2,3 . . .   eq. 11

Where:

-   -   T_(s) is the sampling clock frequency of the digital waveform        generator 150;

${f_{p} = {{\frac{1}{T_{p}}\mspace{14mu} {with}\mspace{14mu} T_{p}}T_{s}}};$

and

-   -   T_(p) represents the period of the alternating +M, −M, +M, −M .        . . pattern.

The digital a waveform generator 150 outputs a sinusoidal tone that isphase-modulated by the alternating pattern +M, −M, +M, −M . . . patternto a digital-to-analog converter (DAC) 152, which converts it to ananalog wave that is passed to a radio frequency (RF) upconverter 154.The RF upconverter 154 up-converts the analog wave to a radio frequencysignal that is broadcast using an antenna 156 in a manner known in theart.

FIG. 7 is a schematic diagram illustrating an exemplary circuit forimplementing the digital waveform generator 150 shown in FIG. 6. Aprogrammable value ΔP is input via 160 to a frequency register 162 of anumerically controlled oscillator (NCO) 150. ΔP is defined as:

$\begin{matrix}{\Delta \; P\frac{2^{J} \times f_{1}}{f_{s}}} & {{eq}.\mspace{14mu} 12}\end{matrix}$

Where:

-   -   f₁ is the frequency of a single tone that will be up-converted        to the pilot tone;

${f_{s} = \frac{1}{T_{s}}};$

and

-   -   J is typically ≧24

Output of the frequency register 162 is fed to a phase accumulator 163.The phase accumulator includes a digital summer 164 that adds the output(ΔP) to feedback 165 of a phase register 166, which introduces a1-sample delay (Z⁻¹) to the output of the digital summer 164. The sumoutput by the digital summer wraps around when the sum exceeds a valueof 2^(J)−1. Consequently, the phase register 166 outputs a digitalsawtooth waveform to a lookup address generator 168. The digitalsawtooth waveform has a period T₁, where

$T_{1} = {\frac{1}{f_{1}}.}$

The digital sawtooth waveform is converted within the NCO 158 to adigital sine waveform using the lookup address generator 168, whichgenerates lookup addresses for a lookup table 170. The lookup addressgenerator 168 generates the lookup addresses, for example, by eithertruncating or rounding the output of the phase accumulator 163 to nbits, where, in one embodiment, n=12 or 14. The output of the lookupaddress generator 168 is used by the lookup table to convert the digitalsawtooth waveform to the digital sine waveform, which is fed to a phasemodulator 174. The digital sine wave is represented by:

Sin(2πf ₁ kT _(s)), k=0,1,2,3 . . .   eq. 13

The phase modulator 174 multiplies the sine waveform input by themodulating sequence (b(kT_(s))) to generate the modulating pilot tone,since both the sine waveform and (b(kT_(s))) are sampled at the rate off_(s). Output of the phase modulator 174 is fed to a band pass filter176 with a pass band of about 30 kHz. The filtered signal is conditionedfor input to the DAC 152 (FIG. 6) by an interpolator 178, which performsa sampling rate conversion to convert the sampling rate of the output ofthe BPF 176 to a sampling frequency more suitable for the DAC 152. Theconditioned signal is output via 180 to the DAC 152, which converts thedigital signal to an analog signal as described above with reference toFIG. 6.

FIG. 8 is a schematic diagram illustrating certain components of thewireless microphone 62 shown in FIG. 3. A message module 200 convertswireless microphone input, in a manner well known in the art, to atime-sensitive message content (m(t)) in a message format used by thewireless microphone system 60 (FIG. 3). The message content is passed toan FM modulator 204, which sequentially converts each m(t) into an FMradio signal, also in a manner well known in the art. Simultaneously,the modulating sequence {b} is fed via 206 to a pilot tone generator208, which may be implemented for example as described above withreference to FIGS. 4-7. Output of the FM modulator 204 and the pilottone generator 208 are simultaneously passed to an analog signal summer210, which outputs the combined pilot tone and FM RF signal to theantenna 68 (FIG. 3) of the wireless microphone 62.

FIG. 9 is a flow chart illustrating a method of sensing TV Band channelsin accordance with one embodiment of the invention to find a TV Bandchannel available for use by a TV Band device. In accordance with themethod, the pilot sensor 34 of the TV Band device 32 (FIGS. 1-3) istuned (300) to a particular TV Band channel. The particular TV Bandchannel may be selected in any number of ways, including reference to adatabase of TV Band channels known to be used by DTV broadcasters in agiven geographical area, so that those channels can be excluded from thesearch. Once tuned to the particular TV Band channel, a counter “N” isinitialized to zero (302). The received RF signal is then down converted(304) to an analog intermediate frequency (IF(x(t))) in a manner wellknown in the art. The analog IF(x(t)) is digitized (306) by ananalog-to-digital converter (ND) to produce a digital signal (x(n)).IF(x(t)) is sampled by the A/D at, for example, about a 100 MHz samplingrate, also in a manner well known in the art. The digital signal (x(n))is further down converted/decimated (308) to produce {z(k)} at, forexample, 5.38 MHz

{z(k)} is then sampled at about 20 MHz and a 4096-bin Fast FourierTransform (FFT) is performed (310) on the {z(k)} samples. The energiesof the 4096 FFT bins are accumulated in 4096 registers in a manner wellunderstood by those skilled in the art. The FFT bin size is about 4.88kHz, i.e. 20 MHz/4096=4.88 kHz. The variable N is then incremented (312)by 1, and it is determined (314) if N is greater than a predeterminedvariable “T”. In one embodiment of the invention, T=100. If not, theprocess returns to step 310, and the energy of each FFT bin is againaccumulated as described above. When N>T, a predetermined number of FFTbins around the mandated frequency location of the DTV pilot areselected (316), and the data associated with the selected FFT bins isanalyzed (318) to determine (320) if an energy spike is present thatexceeds an energy spike threshold. The energy spike threshold may be apreset value. In one embodiment the energy spike threshold is set at 0.5dB. Alternatively, the energy spike threshold may be dynamicallyestablished using, for example, a noise floor computed using the energyaccumulated in a predetermined number of the FFT bins, an example ofwhich will be explained below in more detail with reference to FIG. 8.

If it is determined (320) that the TV Band channel does not contain anenergy spike that exceeds the energy spike threshold, the TV Bandchannel is declared (322) to be TV white space that is available foruse, and the process ends. If it is determined at 320 that an energyspike that exceeds the energy spike threshold is present, the energyspike may be a spurious signal or the TV Band channel may be in use andfurther analysis is required. Consequently, the signal is furtherdown-converted to baseband (323) and the baseband signal is examined inthe time domain in a manner known in the art to determine whether thereceived signal contains a DTV Field-sync or a Segment-sync component,which are identifiable components of the baseband signal if the channelis in use by a DTV transmitter. If a DTV Field-sync and/or Segment-syncis identified, the TV Band channel is declared to be occupied (326) by aDTV broadcast, and it is determined (328) if all TV Band channels havebeen searched. If all TV Band channels have not been searched, theprocess returns to step 300 and the search resumes. Otherwise, theprocess ends.

If it was determined at step 324 that neither a DTV Field-sync nor a DTVSegment-sync signal were identified, {z(k)} is correlated with {b(k)}(330) and it is determined (332) if there is a good level of correlationbetween {z(k)} and {b(k)}. If there is a good level of correlationbetween {z(k)} and {b(k)}, the TV Band channel is declared (334) to beoccupied by a wireless microphone system and the process returns to step328. If not, it is assumed that the energy spike was a spurious signal,the TV Band channel is declared (336) to be available for use, and theprocess ends.

The energy spike threshold in accordance with the method described abovewith reference to 320 of FIG. 9 may be computed as described inapplicant's co-pending U.S. patent application Ser. No. 12/543,259 filedon Aug. 18, 2009, the specification of which is incorporated herein byreference in its entirety.

FIG. 10 is a schematic diagram of the pilot sensor 34 of the TV Banddevice 32 shown in FIGS. 1-3, in accordance with one embodiment of theinvention. A tuner 502 connected to the antenna 36 tunes to a particularTV Band channel under the control of a white space manager 518, whichselects the particular TV Band channel in accordance with apredetermined algorithm which is beyond the scope of this invention. Thetuner 502 down-converts the 6 MHz TV signal to an intermediate frequency(IF) signal x(t), as explained above with reference to FIG. 9. Thedown-converted IF signal x(t) is passed to a digitizer 506, in thisembodiment an analog-to-digital converter that converts the IF signalx(t) to a digital intermediate frequency IF signal x(n), which is passedto a downconverter 508. The downconverter 508 down-converts the IFsignal x(n) to another intermediate frequency IF signal {z(k)}, which ispassed to a Fast Fourier Transform (FFT) 510.

The FFT 510 transforms {z(k)} in a manner known in the art, andaccumulates a predetermined number of transform results in FFT bins (notshown), which are well known in the art. Contents of the FFT bins arepassed to an analyzer 512, which may be embodied in the white spacemanager 518. The analyzer 512 analyzes a predetermined number of the FFTbins centered at the ATSC pilot position to determine whether theycontain the energy spike, as explained above with reference to FIG. 9.If so, the analyzer 512 further down-converts the IF to baseband andanalyzes the baseband signal in the time domain to determine if an ATSCfield-sync or segment-sync are present. All search results are reportedto the white space manager 518. If the analyzer finds an energy spike atthe ATSC pilot position, but does not find a field-sync or asegment-sync in {z(k)}, a correlator 514 is correlates {z(k)} with thepredetermined waveform known to be broadcast by wireless microphonesystems. The correlator outputs correlation results to the white spacemanager 518, which makes a final determination about whether theparticular TV Band channel is available white space, as described abovewith reference to FIG. 9.

Although the invention has been described above with reference to awireless microphone system that generates a pilot tone using amodulating sequence, it should be understood that any readilyrecognizable waveform transmitted in the ATSC pilot position can be usedfor the same purpose.

It should therefore be noted that the embodiments of the inventiondescribed above are intended to be exemplary only. The scope of theinvention is therefore intended to be limited solely by the scope of theappended claims.

1. A method of determining whether a radio frequency channel isavailable for use, the method comprising: down-converting a radiofrequency signal in the radio frequency channel to yield adown-converted signal; processing the down-converted signal to generatean ordered plurality of bins; selecting from the ordered plurality ofbins a number of bins centered at a predetermined location of the radiofrequency signal; analyzing the selected bins to determine if an energyspike is present in any one of the selected bins; and if an energy spikeis not present in any one of the selected bins, declaring the radiofrequency channel available for use; else further processing the radiofrequency signal to a baseband signal and analyzing the baseband signalin the time domain to determine if the baseband signal contains a syncsignal.
 2. The method of claim 1 wherein the radio frequency channel isa TV Band channel.
 3. The method of claim 1 wherein the processingcomprises performing a predetermined number of Fast Fourier Transforms(FFTs) of the down-converted signal and the ordered plurality of binsare generated by accumulating an energy into respective bins associatedwith the respective FFTs.
 4. The method of claim 1 wherein thepredetermined location is at a mandated location of an AdvancedTelevision Systems Committee (ATSC) pilot in the radio frequency signal.5. The method of claim 1 wherein the sync signal is a DTV field-sync. 6.The method of claim 1 wherein the sync signal is a DTV segment-sync
 7. Adevice, comprising: a tuner to tune to a channel and receive a signal; adigitizer that digitizes the signal from the tuner to produce adigitized signal; a transform in communication with the digitizer whichtransforms the digitized signal; an analyzer which analyzes thetransformed digitized signal to determine if the digitized signalcontains an energy spike that exceeds a predetermined threshold and ifan energy spike is not present, declaring the particular channelavailable for use, else further analyzing the digitized signal in thetime domain to determine if the digitized signal contains a sync.
 8. Thedevice of claim 7 wherein the transform is a Fast Fourier Transform. 9.The device of claim 7 wherein the device is a TV Band device.
 10. Thedevice of claim 7 wherein the channel is a TV band channel.
 11. Thedevice of claim 7 wherein the sync is a DTV field-sync.
 12. The deviceof claim 7 wherein the sync is a DTV segment-sync.